1.
After David had gone home, we arrived
When we arrived, David had gone home
2.
Before we took a look around the shops, we had lunch.
After we had lunch, we took a look around the shops.
3.
When the light had gone out, we got out of the office
4.
By the time she had explained everything clearly, we started our work
5.
Before my mother came home, my father had watered all the plants in the garden
Before + vế xảy ra sau ( quá khứ đơn), vế xảy ra trước ( quá khứ hoàn thành )
By the time, và after thì ngược lại.
Chúc bạn học tốt !